As a former sysop on an extremely busy system, and as an artist and designer, I've had reason to inform myself very, very well about copyright law in the US as they pertain both to the written word and to artistic creations.

The bottom line is that *all* creative work, in any medium, is *automatically* copyright to the creator, without any need to formally register the work anywhere, or to post a copyright notice. Such registration may *strengthen* one's claim to a particular work, but it is not considered necessary. Even things like your emails and letters to other people are automatically copyright - and so, of course, are photos, musical works, furniture designs - everything.

Under the doctrine of "fair use", a person may use a small portion of another person's work without permission, as long as it basically doesn't use enough of the original that it would dissuade someone from purchasing the original, or would basically reproduce the whole thing. This means you can use short quotations from written works, but not long ones.

It also means that, contrary to what someone else posted here, you *cannot* take another person's work and just change a small part of it and call it your own. You *may* take the small part and utilize it within something entirely new and different, but changing just one small thing while retaining the bulk of the original is a copyright violation just as surely as using the entire original to start with.

The US Copyright Office has full documentation of the above, and more.